Relationships between habitat conditions, larval traits, and juvenile performance in a marine invertebrate

Environmental effects on sets of traits of organisms can have important consequences on populations. In marine species with complex life cycles, plastic responses in larval traits can affect size at metamorphosis and juvenile growth. These traits can ultimately affect survival and recruitment. Here, I evaluate links between pelagic conditions, natural variations in larval physiological traits (duration of development, size of the megalopa stage), size at metamorphosis, and juvenile growth in the shore crab Carcinus maenas. I sampled settling larvae (megalopae) of C. maenas, daily, during the settlement season of three consecutive years, in the island of Helgoland (North Sea, German Bight). Megalopae were brought to the laboratory where postmetamorphic growth was evaluated under different larval food environments. Traits varied considerably at several temporal scales; variation was not consistent among years. However, the size at metamorphosis was always larger at the start of the settlement season. Some variability correlated with predicted changes in transport processes and dynamics of plume fronts around the study area. Positive relationships between predicted periods of formation of plume fronts, settlement rates, and size at metamorphosis suggest a link between oceanographic conditions and individual traits. Size at metamorphosis explained juvenile growth rates: during the first five juvenile stages, larger individuals grew faster and reached larger size at stage (that is, at any stage observed). Megalopae experiencing ad libitum food conditions grew faster as juveniles than those deprived of food, showing that size at metamorphosis and juvenile growth respond plastically to variations in the larval food environment. I conclude that juvenile growth rate in C. maenas can be affected by natural environmental conditions experienced by larval stages; this effect is mediated by changes in the size at metamorphosis. Most likely the most important factor operating on the size at metamorphosis was food (availability/quality). Since juveniles of this species attain a refuge in size from cannibals, changes in size at metamorphosis could codetermine the chances of postsettlement survival. Further consideration of physiological traits and their consequences at the level of populations should lead to a better understanding of how pre-and postmetamorphic processes interact and explain variability in recruitment in species with complex life cycles.